[lektor-619] Fixing https://github.com/lektor/lektor/issues/619, cuased by https://github.com/pypa/virtualenv/issues/1241

assets/install.sh

@@ -24,7 +24,7 @@ if 1:
     import json
     import tempfile
     import shutil
-    from subprocess import Popen
+    from subprocess import CalledProcessError, check_output, Popen
     try:
         from urllib.request import urlopen
     except ImportError:
@@ -121,6 +121,9 @@ if 1:
             os.makedirs(lib_dir)
         except OSError:
             pass
+        try:
+            check_output([sys.executable, './src/virtualenv.py', lib_dir], cwd=t)
+        except CalledProcessError:  # virtualenv 16.1.0, 17+
             Popen([sys.executable, './virtualenv.py', lib_dir], cwd=t).wait()
         Popen([os.path.join(lib_dir, 'bin', 'pip'),
            'install', '--upgrade', 'Lektor']).wait()